How old are you? Back when the game came out, everyone was like, "OMG A NEW SONIC GAME FOR THE FIRST TIME IN FIVE YEARS". Which was an absolute eternity considering during the 8-bit and 16-bit eras, sequels popped out one year after the next for both Mario & Sonic.
In fact, everyone was so happy to finally be getting SOMETHING from Sega that wasn't 3D Blast, they were willing to ignore some very big flaws with the game and essentially tarred and feathered anybody that was critical (me).
Now it's funny how those same issues ended up evolving into the 'Sonic Cycle' and spawned the whole 'return to the classics' movement.
I'm 19, My first playing through this game was like I said 6 years ago or something, If I Remember correct it was in summer 2010, I wasn't really a follower of Sonic but this game got me into it lol, and still I'm not a huge fan of the franchise, Sonic games are just like gaming dessert for me, Nothing I would really pickup seriously.
I enjoyed it for most part, and when I heard there's newer 3D Sonic games, I decided to pick one up, I chose Sonic Unleashed for my PS3, Graphically It seemed better than SADX, But I didn't enjoy it alot because of the linearity of levels, and the Night stages, in which you play as a beast Sonic or whatever, then I found out about Sonic Heroes, my 3rd 3D Sonic games, The level design was cleverer than SU's but It still obliged you to choose Teams, seems like they were trying to Mix 3 Play styles and worry less about making each character go through it's own levels which I didn't really like, I like to go through the whole level with only the character I choose, I couldn't even finish that game because of the slippery controls and the Rail Grinding that liked to stab you on the back (The Canyon level).
So yeah until now SADX is still my favorite 3D Sonic game because it let me play the way I want, the controls are not slippery, and there's not many confusing unnecessary gimmicks (at least not in Sonic, Tails, Knuckles, and Gamma Levels).